1. Field of the Invention
The invention relates to an image forming apparatus which can correct a curl of a sheet.
2. Related Background Art
For example, in an image forming apparatus using an electrophotographic system, generally, there is used a method whereby a toner image formed on a photosensitive drum is transferred to a paper and the toner is fixed onto the paper as a recording material by using a thermal roller. There are also been paper post-processing apparatuses such as a sorter and the like that are connected to such an image forming apparatus, thereby executing post-processes such as sorting, stapling, and the like of papers. When using such post-processing apparatuses, particularly, since a curl state of the paper exerts an influence on stacking performance and a registration performance of the papers, an apparatus using the post-processing apparatuses in combination with a curl correcting apparatus has also been proposed.
Although a curl correction control of the proposed apparatus has a construction such that a correction amount of a curl can be switched in accordance with only the kind of paper such as thickness, size, material, or the like of the paper, no consideration is made with respect to a deposition amount of the toner. There is consequently a case such that a control of a desirable curl correction according to the contents (dense, light) of an image cannot be accomplished.
Even if curl correction control is executed, a defective precision of a stapling position or a defective staple becomes a factor of a poor registration occurs in dependence on a combination of the deposition amount (fixing amount) of the toner and the paper size. Particularly, such defects occur due to the relation between the growing direction of the curl and the spacing direction of the paper. Such a defect occurs when the curl correctable direction and the curl growing direction by the toner differ in dependence on a general curl correcting mechanism which can correct the curl in only the paper conveying direction.
When a curl which cannot be corrected by the curl correcting apparatus occurs in a paper, there is a case where the paper in which the correction of the curl is insufficient is sent to the post-processing apparatus such as a sorter or the like. For example, in an image forming apparatus having a general curl correcting apparatus which can correct only a positive curl (convex downward) of the paper, when images are formed on both of the front and back sides of a paper having a possibility of the occurrence of a reverse curl (convex upward), there is a fear such that the paper in a state in which the reverse curl is not corrected is conveyed to the sorter and a poor registration of the paper and a defective precision of the stapling position and a defective staple in association with the poor registration occur.